If you are certain that you will never want any invertebrates in that container from now on, use a copper solution. It will kill all snails and will leave your fish unharmed as long as you pay attention to the dosage. The dosage is critical. Too much and it will kill your fish, too little and the snails will live. A correct dose will rid you of all snails and not harm your fish. After the control has been used, you can forget, forever, keeping shrimps in that container. It was once said that you could control snails for 2 cents. Put 2 of the 1 cent coins in your filter and all snails would die. These days a 1 cent coin contains tons of zinc and almost no copper so it no longer works but the 2 cent cure worked out great for many of us back in the 60s, 70s and 80s.